Avoid scaling flat loop profiles of vectorized loops

As discussed, when vectorizing loop with static profile, it is not always good idea
to divide the header frequency by vectorization factor because the profile may
not realistically represent the expected number of iterations.  Since in such cases
we default to relatively low iteration counts (based on average for spec2k17), this
will make vectorized loop body look cold.

This patch makes vectorizer to look for flat profiles and only possibly reduce the
profile by known upper bound on iteration counts.

gcc/ChangeLog:

	PR target/110727
	* tree-vect-loop.cc (scale_profile_for_vect_loop): Avoid scaling flat
	profiles by vectorization factor.
	(vect_transform_loop): Check for flat profiles.

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